Current wireless network designs provide for a shared high bandwidth forward channel from a base station to mobile stations for delivering high speed data services. Such a shared channel has been defined for example by wireless standards including 1xEV-DO, 1xEV-DV, MC-DV and UMTS/HSDPA.
In a wireless access network, all users that have acquired the system can be classified into two basic groups: active user group (active user pool) or dormant—user group (dormant user pool). For a user in the active state, it has all necessary dedicated resources allocated for sending and receiving data. Due to the limited resources of a wireless access system, the maximum number of active users supported is restricted to a certain number. All other users then have to be kept in the dormant state. In order to support more active users, some mechanism to allow more operational modes of active state have been proposed. One example is to allow three states—active, control-hold and dormant states. For a user operating on control-hold mode, less resources are consumed than by a user who is operating on active state. In the current 1xEV and UMTS proposals, one fat pipe (e.g., forward packet data channel) is designed for down link packet data transmission. All active mode user's data can be scheduled to be transmitted over the fat pipe. A packet scheduler decides which user's packet among all active mode users is selected to be transmitted based on a scheduling algorithm. A scheduling algorithm, by taking some input parameters, such as each mobile station's real time channel condition, packet deadline, minimum required data rate, fairness, etc., calculates the priority of each user. The user with the highest priority is selected and its packet is transmitted. Typically some mechanism is provided for deciding when to move an active user to a dormant state or vice versa, or to decide when to move a user between the active and the control-hold state. Currently available solutions for state transition control are mainly timer-based. For example, in a timer-based state transition system, for moving an active user out of its active state, a timer is set for an active state user upon its data buffer becoming empty. If some new data arrives before the expiration of the timer, the timer is then reset. Otherwise, the active user is moved from the active state pool at the expiration of this timer. For moving a dormant user to the active state, a FIFO (first-in, first-out) principle is typically employed.
The timer that is employed for such state transitions can be a fixed value (for example as used in 1xRTT) or it can be dynamically changed. The adaptation of the timer value may depend on traffic load, activity of a user's traffic, etc.
There are a number of problems with the above discussed solutions. For example, in respect of moving an active user out of active state, keeping a user with an empty data buffer in the active pool prevents a dormant user with a full data buffer from getting into the active state pool. Keeping an active user with lower scheduler priority in the active state pool may prevent dormant users with high scheduler priority from getting into the active state pool. These together may cause the system to have a higher outage rate and decrease the multiple user diversity gain. In consequence, the overall system throughput and capacity may drop.
For moving a dormant user into the active state, the simple FIFO principle may move a low priority user into the active state while keeping a dormant user with high priority outside the active state. This again may cause reduced system throughput.
In summary, the current state transition control methods are not optimal for packet switching services. This is because they were designed originally for circuit switching services. Lower overall system capacity is provided because more care is given to the users in the active state, and less fairness is given to support all registered users. System throughput in highly bursty traffic mixed cases will also be lower because the number of full-buffer active users is not high enough to provide sufficient multi-user diversity.
Referring to FIG. 1, shown as a very simple wireless network in which a single BTS (base station transceiver) 10 together with two mobile stations MS 12, MS 14. The forward channel 18 is a shared data channel which is transmitted by the BTS 10 to the mobile stations 12, 14, but at a given instant data is only sent to one of the two mobile stations using the shared data channel 18. Each mobile station feeds back information to the BTS 10 to enable it to make decisions as to which mobile station should receive access to the shared data channel at a given instant. In the illustrated example, channel 14 from MS 12 to BTS 10 sends C/I, pilot information and acknowledgement information for MS 12, and channel 16 sends C/I, pilot information and acknowledgement information for mobile station 14.
In conventional 2-state MAC systems, only active and dormant users are allowed. There is a maximum number of active users each of which are given a dedicated resource at full-rate.
In 3-state MAC systems, there is still the active state in which there is a maximum number of users each of which transmits on reverse channel at full-rate, and there is a control-hold state in which users transmit on the reverse channel at a reduced rate. Typically there would be a maximum number of control-hold users, and a dedicated resource is provided but at a gated rate. Finally there is the dormant state in which there is simply a common resource.
MAC state transitions control the allocation/release of dedicated resources, the assignment of full/gated-rate operation.
Referring again to FIG. 1, in the three state system, a mobile station in the dormant state would not send any C/I or pilot information back to the BTS. A mobile station in active state would generate these at the maximum defined rate. For example, these might be sent back to the BTS every 1.25 ms. Control-hold users would then send this information back to the BTS at a reduced rate, for example ¼ as often. This would result in the C/I, pilot being sent back every 5 ms for example. All of these signals sent back by the mobile station to the BTS generate interference for other users, and as such it is desirable to send them as infrequently as possible. Users in the active state generate the highest interference.
There are many different scheduler designs for deciding which user gets the access to the shared data channel 18. Schedulers make a choice between active users when scheduling the shared data channel. In the simplest example, the scheduler ranks all active users, and selects the top user for transmission on the shared data channel 18. Ranking is performed among active state users only. Schedulers do not consider users outside the active state.